1. Field of the Invention
The present invention generally relates to an optical digitizer for inputting in a computer a coordinate indicated by a pointing object such as a finger, a stylus or a pointing stick (hereafter, generically referred to as a pointer) disposed on a coordinate plane by optically detecting the position of the pointer with an image sensor from a periphery of the coordinate plane. More particularly, the present invention relates to an optical digitizer suitable for constructing a pen computing system in combination with a large-sized flat display panel such as a plasma display device or a liquid crystal display device such that a screen of the flat display panel is superposed with the coordinate plane of the optical digitizer. The present invention also relates to a display apparatus provided with such an optical digitizer. The present invention further relates to an optical stylus configured as a pointer for use in such an optical digitizer.
2. Description of Related Art
Recently, a large-sized plasma display panel (PDP) having a diagonal length as large as 40 inches or more has been developed to a stage of practical utilization. As for a liquid crystal display (LCD), one having a screen of the class of 40 inches has been tentatively fabricated by connecting a plurality of smaller panels together. Such large-sized display devices find convenient applications in making presentations in a conference room or the like by displaying a monitor screen of a personal computer, for example. If a pointing operation or a marking operation on a personal computer is performed by touching the screen with a finger directly or with a stylus rather than by operating a pointing device known as a mouse, the audience before the presentation can observe both the presenter and the screen, thereby obtaining a sense as if presented by use of a blackboard for more effective presentation. Therefore, display apparatuses having the output screen unitized also as the input coordinate plane have been developed by combining a display panel, a digitizer, and a touch sensitive panel.
Conventionally, a so-called stereo method is known as a digitizing method considered comparatively easy to combine with a large-size display, in which two television cameras are used to pick up a light point of a stylus to obtain the position thereof. As shown in FIG. 24, in the stereo method, a coordinate plane 1 is arranged thereon with a stylus 2 which can be operated manually. The stylus 2 is attached at tip thereof with a light-emitting member 24. It should be noted that the coordinate plane 1 is superimposed on a large display panel such as a PDP or LCD. Around the coordinate plane 1, TV cameras 12L and 12R are arranged separately on the left and right sides, respectively. The TV cameras 12L and 12R pick up the light point of the stylus 2, and input the picked up light point into a coordinate computing processor 19 as a video signal. The coordinate computing processor 19 processes the image of the stylus 2 to compute positional information (or positional coordinates), and sends the computed positional information to a personal computer 5. Based on the input positional information, the personal computer 5 generates an image signal, and sends the same to a display panel 6. Based on the received image signal, the display panel 6 displays the positional information of the stylus 2, thereby implementing a real-time pointing operation. It should be noted that the positional coordinates of the stylus 2 can be computed based on triangulation.
Referring to FIG. 25, a side view of the conventional display apparatus shown in FIG. 24 is illustrated. The output screen of the display panel 6 made of a large-sized PDP serves also as the input coordinates plane 1. The stylus 2 is operated on the coordinate plane 1. The stylus 2 has attached at a tip thereof a light-emitting member 24 such as a light-emitting diode (LED). The two TV cameras 12L and 12R pick up the light projected or radiated from the light-emitting member 24. FIGS. 24 and 25 consequently show a typical constitution of the conventional stereo method.
However, the conventional digitizers using TV cameras such as described above are too sensitive to extraneous noise caused by lights such as indoor illumination light and sunlight entered through a room window, thereby causing operation errors. Further, the conventional digitizers, when combined with a display panel, pick up the light radiated from the screen, causing operation errors. The PDP is a display of self light-emitting type and therefore emits a considerable amount of light. As for the LCD of the transmitting type using a back light souce, light is radiated from the back plane through the screen at a considerable intensity. Especially, if the pointer is of a passive type that indirectly projects a light by reflecting an extraneous light, the light quantity projected from the pointer is smaller than that of an active type of the pointer having a light-emitting member and directly projecting a light, so that the detection of the light point is seriously affected by the extraneous light, often resulting in operation errors. Still further, because the light point on the coordinate plane is picked up by the TV cameras from the periphery of the coordinate plane, the TV cameras must be arranged around the display panel. This arrangement imposes many restrictions to the installation due to the view field and contour requirements of the TV cameras, preventing a compact installation from being realized. Yet further, positioning of the TV cameras relative to the coordinate plane is cumbersome and difficult, thereby hampering ease of use of the conventional optical digitizing systems.